Based on studies of women of the Luo and Kikuyu tribes of East Africa, researchers have found that people can carry loads of up to 20 percent of their own body weight without expending any extra energy beyond what they'd use by walking around unencumbered.
But don't strap gear to your head just yet. Head-loading only works well if you've been practicing it for many years:
The subjects in these studies began head-loading as children and had developed a peculiar gait that's one-third more efficient than the one we're likely to use.
For untrained controls who have not had years to strengthen the right muscles and build up spinal bone density, carrying things on your head actually requires more energy than using a backpack.
Link -via Joe Carter | Photo: US Army